The present invention relates to dynamic packings for sealing a shaft which passes through a wall, and particularly to packings of the oil seal type.
In rotating vacuum pumps and compressors it is necessary to dynamically seal the drive shaft which extends through the casing. This is usually achieved by means of a radial sealing ring of commercial design. The lubrication and cooling of the radial sealing ring are effected by oil which passes to the sealing ring from the interior of the pump. As a rule, the pressure at the outside is lower than that within the particular machine. These pressure conditions and the requirements as regards the tightness of the seal determine the position in which the radial sealing ring is fitted, its sealing lip extending in the direction in which pressure increases since the sealing ring will only provide an effective seal in this position. If the ring is fitted in the reversed position, the lip tends to lift away from the shaft and therefore does not maintain a seal.
For the purpose of providing a particularly good seal for such a shaft, it is known to use two radial sealing rings arranged so that their lips extend toward one another, and to fill the gap between the two lips with oil so as to lubricate and cool the rings in a satisfactory manner. Because of viscous flow phenomena in the conical gap formed between each radial sealing ring and the shaft, oil can be carried into the gap. This phenomenon is present even when the shaft, as recommended by manufacturers, is smooth-finished by means of a rolling pass for instance, i.e. even when its surface does not contain very fine grooves resembling a screw-thread.
The resulting pumping action of the rings often results in the build-up of considerable oil pressures of up to several bars in the gap between the two sealing rings, these pressures resulting in increased differential pressure loading and therefore in premature wear and premature leakage through the radial sealing rings.
To avoid these disadvantages it has been proposed to relieve the annular gap of pressure by discharging the pressurizing medium outwardly via a balance hole. However, pressure-relief of this kind cannot be carried out in the case of pumps or compressors that handle toxic or radioactive gases and that therefore must be particularly tightly sealed, since such gases would escape with the outflowing oil. Furthermore, this method of relieving pressure is not suitable in the case of pumps and compressors in which the internal pressure is substantially higher than the external pressure since when, for example, the pressure in the interior of the pump is substantially higher than the atmospheric pressure, the radial sealing ring closer to the interior of the pump opens so that the oil and the medium being conveyed are able to escape from the pump in considerable quantities by way of the balance hole.
These disadvantages can be avoided by the provision of a shaft seal of the type disclosed in German Patent Specification No. 858,483 wherein the gap formed between the shaft sealing rings communicates by way of a duct with the space containing the oil. Thus, the oil emerging from the gap flows back into the oil-containing space without passing into the atmosphere, so that the contamination of the atmosphere with contaminated oil cannot occur.
It is also known to evacuate pumps or compressors for toxic, radioactive or high-purity media prior to their use and to examine them for leaks. Also, evacuation is often carried out when cleaning such equipment for the purpose of degassing the walls and the lubricant. Under these pressure conditions, where the internal pressure of the particular machine is lower than the atmospheric pressure, the shaft seal disclosed in German Patent Specification No. 858,483 is unable to meet the stringent requirements regarding tightness. As soon as the machine is evacuated, negative pressure is also set up in the gap between the sealing rings by way of the balance hole, so that the sealing ring which is closer to the atmosphere opens and atmospheric air is able to enter the machine through the duct.